Unlocking the Rare Earth Elements Potential of Allanite: A Comprehensive Study of Beneficiation and Leaching

Abstract

Allanite is a widely distributed accessory rare-earth silicate mineral, commonly associated with garnet, biotite, and feldspar in igneous and metamorphic rocks, and occasionally found in hydrothermal veins. Containing between 14–33 wt.% total rare earth elements (TREEs), allanite is dominated by light rare earth elements (LREEs) such as neodymium (Nd) and praseodymium (Pr). Despite its considerable potential, the exploitation of allanite has been limited due to its complex mineralogy, processing challenges, and the presence of radioactive elements (Th and U). Recent discoveries of high-grade allanite-rich deposits in Wyoming, USA, have renewed interest in its development. However, systematic studies on allanite processing and REE extraction remain scarce. To date, allanite has mostly been treated as a secondary mineral enriched alongside primary REE minerals, rather than processed independently. This dissertation presents a systematic and comprehensive investigation into the beneficiation and leaching of REEs from Wyoming allanite. In the mineral processing phase, density and magnetic separation techniques were optimized. Allanite concentrates achieved TREE recoveries of 70–90%, with mass yields of 10–25%, corresponding to concentration factors of 2.9–7.7 relative to the original feed. Flotation experiments using sodium oleate and dodecylamine (DDA) as collectors demonstrated poor selectivity, suggesting that future work should focus on reverse flotation strategies and the systematic screening of more effective collectors, including potential synergistic combinations, to selectively separate allanite from silicate gangue minerals. In the chemical extraction phase, acid screening tests revealed that sulfuric acid (H₂SO₄) outperforms nitric acid (HNO₃) and hydrochloric acid (HCl) as a lixiviant, likely due to the formation of stronger sulfate complexes with REE³⁺ ions. Temperature and solid-to-liquid (S/L) ratio were found to have a greater influence on TREE recovery than acid concentration or particle size. Direct acid leaching using 1 M H₂SO₄ at 75 °C achieved TREE recoveries of 80–85% within two hours, without significant gangue mineral decomposition as confirmed by XRD analysis. The leaching kinetics exhibited a two-stage behavior: an initial rapid recovery within the first 10 minutes, followed by a slower phase. Kinetic modeling indicated a mixed control mechanism—surface chemical reaction and diffusion through a product layer—with activation energies of 20.3 kJ/mol for the early stage and 10.8 kJ/mol for the later stage. This behavior was attributed to the preferential dissolution of radiation-damaged metamict allanite initially, with slower dissolution of well-crystallized, acid-resistant allanite in the subsequent stage. To address the challenge of the leaching resistance of well-crystallized allanite, three enhanced leaching techniques were systematically evaluated: ultrasonic-assisted, microwave-assisted, and autoclave leaching. Ultrasonic-assisted leaching proved ineffective, yielding less than 4% TREE recovery even after 60 minutes, indicating that cavitation effects were insufficient to disrupt the robust crystal structure of allanite. In contrast, microwave-assisted leaching demonstrated outstanding performance, achieving approximately 88% TREE recovery within just 5 minutes at 150 °C. This efficiency was attributed to the rapid, selective internal heating induced by microwaves, which likely generated thermal shock and associated microstructural changes. SEM imaging of the leached residue revealed the presence of microcracks that were absent in the untreated feed material, supporting the hypothesis that microwave exposure enhanced mineral surface area and reactivity. Kinetic modeling indicated a shift from mixed control to predominantly diffusion-controlled mechanisms at elevated temperatures. Autoclave leaching also produced promising results, with TREE and Fe recoveries reaching around 90% and 60%, respectively, at 150 °C. Compared to microwave leaching, autoclave leaching was more effective for dissolving dense, homogeneous Fe-bearing minerals such as chlorite–clinochlore and almandine–spessartine, owing to uniform external heating and thermal stress from the surface inward. These findings highlight the critical role of both mineralogical properties and energy transfer mechanisms in the design of effective leaching strategies. Following the acquisition of REE-containing leachate from allanite through acid leaching, further purification is necessary using solvent extraction. This dissertation introduces the development of a Gas-Assisted Microbubble Extraction (GAME) system as an innovative approach to solvent extraction. Although only preliminary tests were conducted, the results are promising and highlight the need for continued experimental and theoretical investigation. Compared to conventional solvent extraction methods that rely on beaker-and-stirrer setups, the GAME system demonstrated superior performance—particularly at elevated aqueous-to-organic (A/O) ratios. It maintained TREE extraction efficiencies of approximately 80% even at an A/O ratio of 20, a condition under which traditional methods failed. This enhanced performance is attributed to improved mass transfer, expanded interfacial surface area, and more effective phase contact enabled by the fine dispersion of gas bubbles. In addition to boosting extraction efficiency, the GAME system significantly reduced organic solvent consumption, offering clear advantages for greener and more sustainable REE production processes. Overall, this dissertation advances both the scientific understanding and practical methodologies for processing allanite, encompassing physical beneficiation and advanced leaching strategies. The findings demonstrate that allanite is a viable alternative source of REEs, contributing to the diversification and sustainability of the global REE supply chain. Furthermore, this work provides deeper insight into allanite's mineralogical behavior and leaching kinetics, while introducing innovative extraction technologies that lay the groundwork for the sustainable development of unconventional REE resources.